1. Field of the Invention
The present invention relates to an active matrix display, typified by an LCD, and a method for driving the display, and more particularly, relates to a technique for driving a transistor and an auxiliary capacitor formed in each pixel of a display.
2. Description of the Related Art
FIG. 17 is a block diagram of a conventional display. The display includes a pixel array 1, a pair of vertical shift registers 2a, and a horizontal shift register 3a. The pixel array 1 includes scan lines X extending laterally, signal lines Y extending longitudinally, pixels P arranged in a matrix so as to correspond to the respective intersections of the scan lines X and the signal lines Y, and auxiliary capacitor lines Xs arranged parallel to the scan lines X. The vertical shift registers 2a are arranged on the right and left sides of the pixel array 1 to simultaneously drive the pixel array 1 on both the sides. In other words, each vertical shift register 2a sequentially applies selection pulses to the scan lines X to select the pixels P row by row. The horizontal shift register 3a supplies a signal VIDEO, of which potential is reversed between a high level and a low level relative to a reference potential COM, to each signal line Y to write the signal with a high or low potential into each pixel P of the selected row. Specifically, each signal line Y is connected to a common video line 3b through the corresponding horizontal switch HSW. The signal VIDEO is externally supplied to the video line 3b. The horizontal shift register 3a sequentially turns on or off each horizontal switch HSW to supply the signal VIDEO to the corresponding signal line Y. An image-quality improvement circuit 5 is connected to the respective signal lines Y.
Each pixel P includes a transistor Tr, a pixel electrode, and an auxiliary capacitor Cs. The transistor Tr connects to the corresponding scan line X and signal line Y and conducts in response to a selection pulse. The pixel electrode is shown by an intermediate node between the transistor Tr and the auxiliary capacitor Cs. The signal VIDEO is written into the pixel electrode through the corresponding conducting transistor Tr. The auxiliary capacitor Cs holds the signal VIDEO written in the corresponding pixel electrode. One electrode of the auxiliary capacitor Cs is connected to the corresponding transistor Tr and pixel electrode and the other electrode thereof is connected to the corresponding auxiliary capacitor line Xs, which is common to the auxiliary capacitors Cs in the same row. The auxiliary capacitor lines Xs are tied together into a bundle. The bundle is held at the predetermined reference potential COM. In other words, the potential of the electrode of each auxiliary capacitor Cs is fixed to the reference potential COM.
The display further has counter electrodes (not shown) facing the respective pixel electrodes, with a predetermined space therebetween. An electrooptic material such as liquid crystal is arranged in the space between the pixel electrodes and the counter electrodes. The counter electrodes are held at the predetermined reference potential COM. On the other hand, the potential of a signal to be written into the pixel electrode is positive or negative relative to the reference potential COM.
FIG. 18 is a schematic diagram showing the N-th stage (N-th row) and the (N+1)-th stage ((N+1)-th row) in the pixel array. As mentioned above, each pixel P includes the transistor Tr and the auxiliary capacitor Cs. One electrode of the auxiliary capacitor Cs is connected to the transistor Tr. The other electrode thereof is connected to the predetermined reference potential COM through the corresponding auxiliary capacitor line Xs. In this description, the other electrode of the auxiliary capacitor Cs may be called a Cs counter electrode.
FIG. 19 is a timing chart explaining a method for driving the conventional display in FIGS. 17 and 18. FIG. 19 shows first and second fields. In the first field, all of the scan lines are sequentially scanned once. In the second field, all the scan lines are again scanned in sequence. The N-th stage (N-th row) in the pixel array will now be described as an example. For an arbitrary horizontal period in the first field, a selection pulse (GATE) is applied to the corresponding scan line, so that the pixels in the N-th row are selected. At that time, the potential of each of the corresponding Cs counter electrodes is fixed to the reference potential COM. In the conventional display, the potential of each Cs counter electrode is always fixed to the reference potential COM irrespective of line sequential scanning. For example, a positive signal relative to the reference potential COM is written into each pixel of the selected N-th row. For the next horizontal period in the first field, pixels in the (N+1)-th row are selected. A negative signal relative to the reference potential COM is written into each pixel in the selected row. For the further next horizontal period in the first field, pixels in the (N+2)-th row are selected. A positive signal relative to the reference potential COM is written into each pixel of the selected row. As mentioned above, in the conventional display, the polarity of a video signal written in each pixel row is generally reversed every horizontal period (1H). This is called 1H reversal driving. In the second field, 1H reversal driving is similarly performed. When the first field is compared to the second field with respect to the same pixel row, it is known that the polarity of the signal in the first field is opposite to that in the second field. In other words, 1F reversal driving is performed. The N-th pixel row will now be described as an example. The positive video signal is written in the pixels in the N-th row in the first field. In the second field, a negative video signal is written into the same pixels.
Japanese Unexamined Patent Application Publication Nos. 11-271787 and 2001-159877 disclose methods for driving the above-mentioned conventional display.
In an active matrix display, generally, each pixel includes a transistor for writing a signal into the corresponding pixel electrode and an auxiliary capacitor for holding the signal written in the pixel electrode. Each of the above-mentioned active and passive devices includes a thin-film device having a thin layer of, for example, silicon. In conventional driving methods, to stably hold a signal in one field, it is desirable that the capacitance of the auxiliary capacitor of each pixel be increased. The increase in capacitance of the auxiliary capacitor prevents light leak in the transistor. On the other hand, the width of a channel of the transistor is narrowed to reduce the leak. Therefore, the resistance of the channel in the transistor is increased. The current drive capacity tends to be restricted. This leads to the limitation of a capacity to charge the auxiliary capacitor. As mentioned above, the conventional technique has inconsistent conditions, namely, the increase in capacitance of the auxiliary capacitor and the increase in resistance of the transistor. Unfortunately, the conventional technique can hardly overcome disadvantages such as insufficient signal writing and a spot defect caused by leak. As the definition of the active matrix display becomes higher, the number of pixels increases more sharply. Write time for each pixel is reduced inversely proportional to the increase in number of pixels. Disadvantageously, image quality is seriously deteriorated due to the insufficient signal writing and the spot defect caused by leak.
To overcome the above-mentioned disadvantages, a common reversing method has conventionally been provided. According to the method, synchronously with 1H reversal driving of a video signal, a potential of each counter electrode (common electrode) is reversed so as to be opposite in phase to that of the video signal relative to a reference potential. Synchronously with reversing the counter electrode, the potential of the Cs counter electrode of each auxiliary capacitor is also reversed. According to the common reversing method, however, the potential of each of the counter electrodes arranged in all of the pixels is changed between a positive level and a negative level every horizontal period (1H). Thus, the extremely large amount of charge is required. Actually, it is difficult to charge or discharge the counter electrodes at a high rate. The common reversing method is not exactly an effective solution.